The diminishing reserves of fossil fuels and the emission of harmful gases connected with their use have increased the interest in utilizing organic materials, especially non-edible renewable resources, for making liquid fuels capable of replacing fossil ones.
One of the existing processes for the conversion of biomass includes pyrolysing lignocellulosic material obtained from biomass to obtain pyrolysis oil. Pyrolysis is a process for thermal conversion of solid fuels in the complete absence or with limited supply of oxidizing agent such as air or oxygen. Commercial applications are either focused on the production of charcoal (slow pyrolysis) or production of a liquid product (fast pyrolysis), the pyrolysis oil. Especially the latter is potentially interesting as a substitute for fuel oil and as a feedstock for the production of synthetic gasoline kerosene, jet fuel or diesel fuel.
During pyrolysis, which takes place at temperatures in the range 400-700° C., most of the cellulose and hemicellulose and part of the lignin will disintegrate to form smaller and lighter molecules which are gases at the pyrolysis temperature. As these gases cool, some of the vapors condense to form a liquid, called pyrolysis oil. The remaining part of the biomass, mainly parts of the lignin, is left as a solid i.e. the charcoal.
Conventional slow pyrolysis has been used for thousands of years for the production of charcoal. Pyrolysis temperatures of around 500° C. are used in the slow pyrolysis of wood, and both solid char and liquid are formed. In fast pyrolysis the biomass is rapidly heated and it decomposes in vapors, aerosols, and some charcoal-like char. The cooling of the vapors and aerosols yields pyrolysis oil that has a heating value that is about half of that of conventional fuel oil.
Depending on the feedstock, fast pyrolysis produces about 60-70 wt % liquid pyrolysis oil, 15-25 wt % of solid char and 10-20 wt % of non-condensable gases. In principal no waste is generated as the char and liquid pyrolysis oil can be used as fuels and the gas can be recycled into the process. However, due to its instability the liquid pyrolysis oil is difficult to use as such. It can be used to replace heavy heating oil in, for example, industrial boilers, but in order to be able to use it as transportation fuel, it needs to be upgraded before use as fuel. Therefore several processes for additional processing of pyrolysis oil have been suggested in the literature.
US 2011/0119994 relates to catalytic hydrotreatment of pyrolysis oil where feed comprising pyrolysis oil is subjected to a hydrodeoxygenation step in the presence of a catalyst. U.S. Pat. No. 7,578,927 discloses diesel production from pyrolytic lignin where pyrolytic lignin is hydrotreated and the hydrocracking unit includes a catalyst. In U.S. Pat. No. 4,795,841 pyrolyzate oil is exposed to hydrogen gas and a suitable catalyst at a temperature in the range of 250° C. to 300° C.
Despite the ongoing research and development of processes for upgrading pyrolysis oil, there is still a need to provide an improved process for treating pyrolysis oil.